(1) Field of the Invention
The present invention relates to a method and an apparatus for lighting a high pressure discharge lamp, a high pressure discharge lamp apparatus, and a projection-type image display apparatus.
(2) Description of the Related Art
High pressure discharge lamps such as high pressure mercury lamps or metal halide lamps generally provide highly bright luminous flux, which is regarded as an excellent advantageous effect thereof. On the other hand, a problem of such high pressure discharge lamps is that they require a long time from a start of discharge until they obtain a luminous flux that has a certain level of brightness (hereinafter, the time required for obtaining such a luminous flux is referred to as “light rise time”). Reducing the light rise time has been a longstanding challenge in the field of the high pressure discharge lamp.
Especially, projection-type image display apparatuses (hereinafter referred to as “projectors”) are desired to have high illuminance recently, and therefore need to use higher-output high pressure discharge lamps than before.
However, high pressure discharge lamps become larger in size as they become higher in power. When this happens, the thermal capacity of the lamp components including the glass bulb increases. This decreases the speed at which the light-emitting metal evaporates in the discharge space, further prolonging the light rise time.
Some lighting methods for motor vehicle high pressure discharge lamps have solved this problem by first supplying the lamps with much higher power than the power at the normal lighting to promote the light rise, then supplying the lamps with a constant power (see, for example, Japanese Laid-Open Patent Application No. H04-349396 (Patent Document 1), Japanese Laid-Open Patent Application No. H08-078175 (Patent Document 2)).
However, if the above-mentioned technologies for motor vehicles are applied to high pressure discharge lamps for projectors, edges of the lamp electrodes would melt. This is a serious damage to the electrodes and lamps.
The motor vehicle high pressure discharge lamps are standardized by Japan Electric Lamp Manufacturers Association (JELMA) or the like, and they are designed to have leeway in electrode size relative to the lamp power. As a result, even if a much higher power than the power at the normal lighting is supplied to the lamps for motor vehicles, the lamp electrodes do not suffer much damage. Also, in the lamps for motor vehicles, a xenon gas is introduced at a pressure as high as 500 kPa or more. This causes the resistance component in the discharge space to be high, restricting the lamp current to a level not so high. For these reasons, electrodes of the motor vehicle high pressure discharge lamps do not suffer damage with use of the lighting methods disclosed in the Patent Documents 1 and 2.
However, high pressure discharge lamps for projectors are required to provide high brightness to maintain the screen brightness.
To provide high brightness, it is essential to maintain the electrode edges at high temperatures to increase the arc temperature. For this reason, the size of the electrodes needs to be reduced as much as possible to reduce the thermal capacity. Also, to increase the converging efficiency of the concave reflecting mirror, a point light source is preferred. However, reduction in the distance between electrodes leads to a larger amount of lamp current flowing there. Also, supply of an excessively high power may lead to an excessive increase of the lamp current.
Accordingly, if a power much higher than a power at a normal lighting is supplied to a high pressure discharge lamp for a projector under the same condition as a high pressure discharge lamp for a motor vehicle, the temperature at edges of electrodes would increase abnormally, melting the electrodes. When this happens, an appropriate point light source is not obtained, and in the worst case, a lighting failure occurs.
One might think that a rare gas such as xenon may be introduced at a high pressure into the projector high pressure discharge lamp to increase the resistance in the discharge space, as in the motor vehicle high pressure discharge lamps. However, as described earlier, high pressure discharge lamps for projectors are relatively large in size, and therefore it is difficult to introduce a rare gas at a pressure higher than a certain pressure, from a viewpoint of the manufacturing technology.
Due to the circumstances described above, in conventional lighting methods for high pressure discharge lamps for projectors, to prevent the electrodes from suffering damage, a constant current control is performed so as to supply an electric current, which does not exceed the maximum value of a current which is supplied while the constant power control is performed, for a duration of the initial lighting period between a lighting start and a constant power control. In such circumstances, it is extremely difficult to reduce the light rise time.